Various forms of therapy can be applied within the body of a human or other mammalian subject by applying energy from outside of the subject. In hyperthermia, ultrasonic or radio frequency energy is applied from outside of the subject's body to heat the tissues. The applied energy can be focused to a small spot within the body so as to heat the tissues at such spot to a temperature sufficient to create a desired therepeautic effect. This technique can be used to selectively destroy unwanted tissue within the body. For example, tumors or other unwanted tissues can be destroyed by applying heat to heat the tissue to a temperature sufficient to kill the tissue, commonly to about 60.degree. to 80.degree. C., without destroying adjacent normal tissues. Such a process is commonly referred to as "thermal ablation". Other hyperthermia treatments include selectively heating tissues so as to selectively activate a drug or promote some other physiologic change in a selected portion of the subject's body. Other therapies use the applied energy to destroy foreign objects or deposits within the body as, for example, in ultrasonic lithotripsy.
Magnetic resonance is used in medical imaging for diagnostic purposes. In magnetic resonance imaging procedures, the region of the subject to be imaged is subjected to a strong magnetic field. Radio frequency signals are applied to the tissues of the subject within the imaging volume. Under these conditions, atomic nuclei are excited by the applied radio frequency signals and emit faint radio frequency signals, referred to herein as magnetic resonance signals. By applying appropriate gradients in the magnetic field during the procedure, the magnetic resonance signals can be obtained selectively from a limited region such as a two-dimensional slice of the subject's tissue. The frequency and phase of the signals from different portions of the slice can be made to vary with position in the slice. Using known techniques, it is possible to deconvolute the signals arising from different portions of the slice and to deduce certain properties of the tissues at each point within the slice from the signals.
Various proposals have been advanced for using magnetic resonance to monitor and guide application of energy within the body. As disclosed, for example, in the U.S. Pat. Nos. 4,554,925, 4,620,546 4,951,688 and 5,247,935, the disclosures of which are hereby incorporated by reference herein, certain known magnetic resonance procedures are temperature sensitive, so that magnetic resonance data acquired using these procedures will indicate changes in temperature of the tissues. For example, a magnetic resonance parameter referred to as T.sub.1 or spin-lattice relaxation time will vary with temperature. If magnetic resonance imaging apparatus is actuated to acquire T.sub.1 for various volume elements or "voxels" within the subject, the data for different voxels will vary with temperature, at least within a tissue having generally the same composition. The data can be portrayed as a visible image and hence different temperatures can be shown by the differences in brightness or color within the displayed image. Thus, the location within the body being heated can be monitored by monitoring such a visible image during application of energy to the body. Also, the degree of the heating can be monitored by monitoring T.sub.1 for the heated regions. Magnetic resonance parameters other than T.sub.1 can be portrayed or monitored in the same way.
Although these procedures have well been known, they have not been widely adopted in the medical community. Magnetic resonance imaging instruments of the types commonly used for medical diagnostic applications include large, precise magnets which are arranged to impose a high magnetic field, typically about one Tesla or more over a relatively large imaging volume typically 10 cm or more in diameter. Certain magnetic resonance imaging static field magnets severely limit access to the subject. For example, a solenoidal air-core superconducting magnet may have superconductive coils surrounding a tubular subject-receiving space. The subject lies on a bed which is advanced into the said tubular space so that the portion of the patient to be imaged is disposed inside of the tubular space. Iron core magnets typically have ferromagnetic frames defining opposed poles and a subject-receiving space lying between the poles. Permanent magnets or electromagnets are associated with the frame for providing the required magnetic flux. Depending upon the design of the magnet, either the superconductive coils or the frame may obstruct access to the patient during operation of the magnetic resonance instrument. Moreover, because the magnetic resonance imaging instruments typically employed in medicine are expensive, fixed structures, there are substantial costs associated with occupancy of the instrument. Because hyperthermia procedures typically require significant time to perform, it is expensive to perform these procedures while the patient is occupying the magnetic resonance imaging instrument. Moreover, because instruments of this type are typically found only in specialized imaging centers and radiology departments of hospitals, use of the magnetic resonance imaging instrument for therapeutic procedures is associated with considerable inconvenience to the patient and to the treating physician. Thus, despite all of the efforts devoted heretofore to MRI-guided hyperthermia procedures and apparatus, there remains a considerable, unmet need for improvements in such procedures and apparatus which would reduce the cost and increase the convenience of such procedures.
Moreover, there has been a need for further improvement in hyperthermia procedures of this type. The physician typically aims the energy-applying device manually and applies so-called "subthreshold" doses of energy, sufficient to heat the tissues slightly but insufficient to cause permanent change in the tissue. The physician then observes the location of the heated spot on a magnetic resonance image to confirm that the energy-applying device is aimed at the desired location in the subject's body.
The response of the tissues within the body to the applied energy varies. Differences in tissue properties such as specific heat and thermal conductivity will cause differences in the change in the temperature caused by absorption of a specific amount of energy. The "susceptibility" or tendency of the tissues to absorb the applied energy also varies from place to place. Therefore, after the device has been aimed onto a particular spot, the physician must apply a therapeutic dose by gradually increasing the amount of the energy applied to the spot and monitoring the degree of temperature change to the spot by means of the magnetic resonance information as, for example, by observing the visually displayed magnetic resonance image.
Typically, the spot heated during each operation of the energy-applying device is relatively small as, for example, a spot about 1 mm-3 mm in diameter. To treat a large region within the subject, the spot must be repositioned many times. All of this requires considerable time and effort. Moreover, the procedure is subject to errors which can cause damage to adjacent organs. For example, thermal energy is commonly applied to treat benign prostatic hyperplasia or tumors of the prostate gland. If the physician mistakenly aims the energy-applying device at the urethra and actuates it to apply a therapeutic dose, the delicate structure of the urethra can be destroyed. Therefore, improvements in thermal energy treatments which improve the safety of such treatments and reduce the effort required to perform such treatments, would be desirable.